This invention relates to vibration resistant electrochemical cells wherein a generally tubular cell case is inwardly deformed to engage and restrain the cell's electrode assembly against movement.
It is, in many instances, important that sealed electrochemical cells be able to withstand physical jarring, shocks and mishandling. It is additionally important that the cells maintain their physical and electrical characteristics and integrity during the manufacturing process and, thereafter, throughout service.
Electrochemical cells of the sealed type include an electrode assembly disposed within a cell casing. To a great extent, the cell's physical and electrical integrity is ensured by assembling the electrode assembly to a close fit or tolerance between the casing so as to preclude relative movement between the two when the cell is in service. When a firm and uniform pressure is applied to the adjacent positive and negative electrodes of the electrode assembly, performance of the cell is generally improved.
With electrodes that are spirally wound or coiled, wherein initially flat, positive and negative electrode plates are stacked alternately upon one another and then wound in a tightly packed coil, certain manufacturing difficulties may be encountered. First, after they are wound, the electrodes have a tendency to uncoil slightly and thus expand, either prior to or after insertion of the electrode assembly into the casing. Additionally, it is difficult to wind electrodes precisely during mass production so as to obtain a close, uniform tolerance from one electrode assembly to the next. For both of these reasons, endeavoring to achieve a uniformly close fit or tolerance between the electrode assembly and the cell casing may be impracticable.
One method of overcoming the foregoing difficulties is described in Deschamps U.S. Pat. No. 3,364,069 (1968). There, the spirally wound assembly of positive and negative electrodes and interleaved separators is placed into a cylindrical cell casing which, following closure, is permanently deformed by a plurality of grooves impressed in the casing wall. These deformations of the casing wall take the form of either axially spaced annular grooves, a spiral groove about the casing, or a series of grooves extending longitudinally of the cell axis. Although these measures do adequately avoid both undesired unwinding of the electrode assembly and movement of the assembly within the case during service, a major disadvantage is that undue stresses are placed upon the coiled electrode assembly by the grooving steps. Specifically, stresses occur at the outer convolutions of the electrode assembly where the coiled electrode plates terminate. At these locations, the terminal ends of the electrode plates present relatively sharp edges or abrupt discontinuities which result in the creation of high shear forces upon the outer convolutions of the electrode plates when the casing is inwardly deformed. With time, these edges cut through the interleaved separators and cause internal shorting between either the electrode plates or between the electrodes and the case itself. Even though such damage does not appear in every manufactured cell, the reject rate of cells off the assembly line increases, as does the probability of later failure.
The primary object of the present invention is to provide a cell, and a method of manufacturing the cell, which overcomes the disadvantages of the prior art techniques for constructing vibration-resistant cells.
Among the further objects is the provision of a structurally new vibration-resistant electrochemical cell.